Given the non-existence of a material capable of efficiently detecting both an infrared radiation and a visible radiation, bispectral detection in these two spectral bands is usually achieved by means of two sensors. Each sensor is dedicated to a particular radiation type and is placed in a specific optical path.
In fact, this type of bispectral detector requires an adjustment of the optical centers and axes of the two optical paths to ensure that the sensors observe the same scene.
This type of adjustment, denoted by the term “spatial harmonization”, is generally quite tricky. Moreover, it needs to be repeated on a regular basis on account of vibrations or impacts sustained by the detector, which misalign the optical paths.
To overcome this drawback, a bispectral detector has been designed wherein a photoelectric element array is positioned underneath a bolometric micro-bridge array, to form a bispectral detection element array. Such a detector is described for example in the document U.S. Pat. No. 6,320,189.
This detector, whereof one bispectral detection element is shown in FIG. 1, includes a CMOS or CCD multiplexer circuit 1, formed in a substrate, an insulating layer 2 covering the circuit 1, and a bolometric micro-bridge 4 for detecting the infrared radiation.
The micro-bridge 4 includes a thin bolometric layer 9, made absorbent to infrared radiation, electrodes 7, extended by thermal insulation arms 8, and supports 5 of the layer 9 and electrodes 5, providing support thereto and electrically connecting the electrodes 7 to the multiplexer circuit 1 via metal connections 6.
A reflecting layer 3 is also provided underneath the layer 9, to form therewith a quarter-wave plate, increasing the sensitivity of the bolometric detector to infrared radiation.
A photoelectric element 10, sensitive to visible radiation, such as a photodiode or a phototransistor for example, is formed furthermore in the substrate of the multiplexer circuit 1 and placed under the micro-bridge 4.
The thin bolometric layer 9, the electrodes 7, the reflecting layer 3 and the insulating layer 2 are made out of materials that are at least partially transparent to visible radiation in order to let through some of this radiation so that it is detected by the photoelectric element 10.
The detector so constituted is placed in the focal plane of an optic, with the micro-bridges 4 placed towards the optic.
A radiation comprising an infrared (IR) component, and a visible (VIS) component and incident, via the optic, on the detector on the micro-bridge 4 side, then sees its IR component picked up thereby, while the VIS component part, which has successfully penetrated as far as the photoelectric element 10, is picked up by the latter.
Thus, for this type of bispectral detector, a single optical path is necessary and all the problems associated with spatial harmonization are avoided.
However, this type of detector suffers from intrinsic faults which are detrimental to the quality of the bispectral detection.
Indeed, some of the visible radiation is necessarily absorbed by the materials constituting the bolometric detector. Visible light detection efficiency is thus weakened.
Moreover, it is necessary to choose for the bolometric detector materials that are at least partially transparent to visible radiation. In fact said choice of material is not necessarily optimum for the detection of IR radiation.
Furthermore, this type of stacked array detector requires a quality, and therefore expensive, optic, in order to be able to focus the infrared or visible radiation in the restricted area of the space containing the two arrays.
Lastly, since the photoelectric element array and the micro-bridge array are formed in different planes that may be more than three micrometers apart in some applications, it is difficult to focus at the same time on both arrays using a single optic. The result of this is therefore a loss in terms of resolution.
The purpose of the present invention is to resolve the aforementioned problems by proposing a bispectral detector that does not require one optical path per detection type, while providing optimum efficiency and resolution for each detection type.